1. Field of Invention
The present invention relates to a timepiece with an internal antenna, and more particularly to a timepiece with a built-in patch antenna (microstrip antenna) that receives radio frequency signals transmitted from an external source.
2. Description of Related Art
Timepieces that have an antenna for receiving radio frequency signals are known from the literature. Included in these RF signals are satellite signals transmitted from positioning information satellites, for example.
A wristwatch that has a patch antenna for receiving satellite signals as taught in JP-T-2007-526985 (Japanese translation of PCT international application JP-A-2007-526985) is an example of a timepiece with an internal antenna.
More specifically, JP-T-2007-526985 describes a wristwatch case rendered by a cylindrical metal external case member with a bottom. A bezel made of a material (such as plastic) through which RF signals pass easily is connected to the top side (open side) of the external case member, and a crystal is disposed inside the bezel.
With this wristwatch, the drive staff of the hands that display the time is disposed in the center of the surface of the dial. When the case is metal, the patch antenna inside the case is preferably disposed as close as possible to the face side of the timepiece, that is, the dial, so that RF signals can be received through the face side of the timepiece.
As a result, the patch antenna must be disposed between the drive staff and the inside circumference surface of the case. The patch antenna described in JP-T-2007-526985 is therefore disposed to a position offset from the plane center of the case toward the 12:00 o'clock position of the dial.
Because the patch antenna is a flat antenna, the thickness can be reduced and increase in the timepiece thickness can be suppressed when the patch antenna is incorporated inside the wristwatch.
However, in order to ensure the necessary reception performance, the patch antenna must have a plane area of a certain size. Recent technological advances in dielectric materials and radio sensitivity have enabled reducing the size of the patch antenna to some degree, but further reduction in size is complicated by the need to ensure the required reception performance. The plane area of a wristwatch must also be kept to a size that enables wearing on the wrist.
Therefore, when the patch antenna is located between the inside circumference surface of the case and the drive staff located in the plane center of the timepiece, the patch antenna is disposed adjacent to the case as shown in FIG. 1 and FIG. 2 in JP-T-2007-526985.
However, signal reception by the patch antenna will be easily affected if a case made from metal or other conductive material is adjacent to the side of the antenna (the outside circumference side facing the inside circumference surface of the case), and reception performance will drop. Reception performance is therefore assured in JP-T-2007-526985 by extending the bottom edge of the bezel to near the bottom surface of the patch antenna so that the side of the patch antenna is covered by the bezel.
In addition to practical functionality such as displaying the time and communication, however, timepieces, and particularly wristwatches, also need an appearance of quality. Such timepieces therefore generally use a metal material on the outside of the case.
However, a problem with the timepiece taught in JP-T-2007-526985 is that the plastic bezel occupies a large area and the quality of the timepiece therefore appears lower.
On the other hand, if a metal case is used to improve the appearance of quality, reception performance drops because the patch antenna is adjacent to a metal case as described above. More specifically, because the patch antenna is unidirectional, it is preferable to an non-directional antenna when RF signals are received through the open side of the metal case, that is, from the crystal and dial side of the timepiece.
Even so, because the patch antenna works on the same principle as a slot antenna and the strongest signals are emitted from the gap between the end of the top conductor (antenna end) and the ground plane, performance drops sharply if metal is close to the side of the patch antenna. A significant drop in the reception performance of the antenna is therefore a problem.